Many wireless communication systems now support multiple kinds of services, including for instance circuit switched voice services, packet data services, high data rate services, etc. These different services have very different characteristics. Moreover, different applications using the same general service may nonetheless impose different demands on that service. For instance, an internet browsing application may be supported by a packet data service that has a variable delay and throughput, while a multimedia streaming application may be supported by a packet data service having a relatively constant average throughput and low delay.
A wireless communication system supports these varying services through the use of radio bearers. A radio bearer supports the transfer of user data over a radio connection between a wireless communication device and a base station with defined data transfer characteristics (e.g., with a defined quality of service, QoS). Different radio bearers are configured to provide different defined transfer characteristics.
Under some circumstances, though, the configuration or state of a given radio bearer may need to be changed, e.g., in order to optimize the radio bearer for the current requirements of the wireless communication device. A change in the configuration or state of a radio bearer involves, as non-limiting examples in a context where the system is a High Speed Packet Access (HSPA) system, adding or removing the radio bearer, moving the radio bearer between a dedicated physical channel (DPCH) and enhanced uplink (EUL)/high speed (HS), changing the spreading factor and/or bit rate, and/or adding or removing connection capabilities (e.g., EUL 2 ms/10 ms transmission time interval (TTI), Dual Cell or multi-carrier, 64 quadrature amplitude modulation (64QAM), multiple-input multiple-output (MIMO), continuous packet connectivity (CPC), downlink (DL) enhanced layer 2 (L2), uplink (UL) improved L2).
Consider the specific example of a radio bearer configuration change relating to a change in the TTI of a radio bearer (i.e. a TTI switch). The TTI is a radio bearer parameter that defines the interval of time in which a transmission occurs over the air interface. In some systems, for instance, a set of one or more so-called transport blocks are fed from a medium access control (MAC) layer to the physical layer, and the TTI is the time it takes to transmit that set of one or more transport blocks over the air interface.
Regardless, a longer TTI (e.g., 10 ms or above) proves more robust in the face of poor channel conditions. On the other hand, a shorter TTI (e.g., 2 ms) reduces latency, which is required to provide good end-user experience when supporting mobile broadband services. Because of this, it is desirable to use a shorter TTI over as wide an area as possible. However, at least in current third generation (3G) networks, a substantial number of large macro cells still exist. With a macro cell being so large, it generally proves challenging for the cell to support a TTI as short as 2 ms over its entire coverage area. In such environments, it may be necessary to fall back to a longer TTI, e.g., 10 ms, when a wireless communication device approaches the cell boundary. This however requires that a radio bearer configuration change be triggered when the device approaches the cell boundary, and that the change be applied.
In the current third generation partnership project (3GPP) specifications (i.e. up to Rel-11, meaning Release-11), when enhanced dedicated channel (E-DCH) is used in CELL_DCH state, it is the radio network controller (RNC) that configures if a user should operate on 2 ms TTI or 10 ms TTI. Here and in the following description, the concept of “user” is to be understood as being equivalent to “wireless communication device”.
During the ongoing 3GPP Rel-12 (meaning Release-12) discussion, with particular regard to the Study Item “Study on Further EUL Enhancements”, several methods are being evaluated, aimed at increasing the EUL coverage especially for 2 ms TTI operations.